_a._ The “water piston with fibrous packing” is preferred for many situations in the West or South, or for water containing grit or mud, like that of the Ohio River; and, for the comparatively few cases where pump pressure governors are used, the packed piston will give better service and longer wear. _b._ The removable bushing or cylinder in which this piston works must be of solid bronze. _c._ As stated in Art. 28 _d_, this bushing should be so constructed as to be readily interchangeable with the bushing of the inside plunger type. _d._ The length of bushing must be such that the ends of piston will barely come short of the edges of cylinder at contact stroke and not uncover. _e._ The thickness of the cylindrical bushings must be not less than is given in the following table: BUSHINGS FOR PACKED WATER PISTONS. ====================+========+=======+=======+======== =Size of Pump.= | =500= | =750= |=1000= | =1500= | =gal.= | =gal.=|=gal.= | =gal.= --------------------+--------+-------+-------+-------- =Solid Bronze.= | | | | | | | | Thickness at |7/16-in.|1/2-in.|1/2-in.|9/16-in. extreme end | | | | | | | | Tapered evenly from |9/16 „ |5/8 „ |11/16 „|3/4 „ end to a thickness | | | | next to bearing | | | | of not less than | | | | | | | | Thickness at center |3/4 „ |3/4 „ | 3/4 „|13/16 „ bearing at least | | | | --------------------+--------+-------+-------+-------- _f._ In other respects, the specifications for plunger bushings, already given in Art. 28, will apply to the above. _g._ The water piston used in the shell described above must expose not less than 2 inches in width of fibrous packing, and must be of bronze, with disc and follower accurately turned to a sliding fit, so that the leakage past it will be a minimum, even when no fibrous packing is in place. There must be at least 2 inches in length of metallic bearing on both disc and follower. The follower must be accurately centered and fitted to hub of piston, so that alignment will not be disturbed if taken apart. _h._ The water piston must be of simple and strong construction, with follower bolts tightly fitted, and with fibrous packing so cut as to prevent by-passing. _i._ All materials used in construction of piston, except packing, must be brass, bronze, or other non-corrosive metal. _j._ Bushing studs must be of Tobin Bronze, and of such size and number, that the maximum stress at the bottom of the screw thread shall not exceed 10,000 lbs. per square inch, in the event of plunger becoming fast in the bushing with 80 lbs. of steam in the steam cylinders. _k._ For each bushing stud there must be provided a composition nut and check nut.